There were really only 3 settings that stood head and shoulders above the rest during the 3e era:
Midnight- for reasons already described. I'd LOVE to see a 4e Midnight, but I doubt it will happen. 4e actually fits the Midnight setting better than 3e did, due to the fact rituals could be used by anyone with the proper training, and the reduced reliance on magic items. Hmmmm..... I might have to try this on my own.
Arcanis- probably the best overall setting during the 3e era, it took aspects of fantasy familiar to everyone, and then tweaked and modified them to make a truly unique and very interesting world. Again, I'd love to see Arcanis converted to 4e
Freeport- because undead pirates, Lovecraftian cults, and the setting are just too cool!
An honorable mention goes to Dawnforge- it looks great, but I never have gotten to play it yet, and it didn't get the support Midnight did from FFG.